1. Field of the Invention
This invention relates to a distance measuring device and more particularly to a distance measuring device which is to be used in an automatic focusing device for improving the accuracy of the distance measurement.
2. Description of the Prior Art
Heretofore, many distance measuring devices for automatic focusing arrangement of photographic cameras have been proposed. The majority of them are arranged to operate on the principle of the trigonometric distance measuring method. In a typical example, a light projecting optical system and a light receiving optical system are arranged to have their optical axes spaced as much as a base line length. Light flux projected from a light projecting element included in the light projecting optical system is irregularly reflected as it impinges on an object the distance to which is to be measured. With the light flux thus reflected, an image of the projected light spot on the object is formed on a light receiving or light sensitive element. This imaging position varies with the distance to the object so that information on the distance to the object can be obtained from the imaging position. In the case of a camera, a lens group which performs a focusing action within a photo-taking optical system is interlocked with the aforesaid light sensitive element (and also with the light projecting element) through a cam mechanism or the like in such a way as to permit detection of the distance to an object to be photographed, and thus to permit the automatic focusing of the photo-taking optical system.
However, referring to FIGS. 1(A) and 1(B) of accompanying drawings, an actual camera must be provided with other operational members such as a member for manual zooming, etc. Then, the arrangement of such members compels the light projecting element 1 and the light sensitive element 2 to be arranged in offset positions away from the photo-taking lens 3.
FIGS. 1(A) and 1(B) shown by way of example an 8 mm motion picture camera or a video camera C incorporating therein an automatic focusing device arranged to operate in accordance with the trigonometric distance measuring method. In this case, the camera is provided with a bar 4 for manual zooming. The bar 4 can be turned within a region "a". The operator holds a grip 5, for example, with his right hand to operate a trigger button 6 and performs manual zooming with his left hand. In order to prevent the front parts of the light projecting element 1 and the light sensitive element 2 from being covered by the left hand, the elements 1 and 2 must be arranged away from the center of the photo-taking lens 3 as shown in FIG. 1(A). With the camera arranged in this manner, focusing by scanning either with the light projecting element 1 or with the light sensitve element 2, for example, with only the light sensitive element 2, causes the light measurement position to vary within the view finder image plane shown in FIG. 2.
Assuming that when the distance measurement position (or the position of an image of the projected light) is identified by portion 8-1 of the whole image plane 7 as shown in FIG. 2, for a certain distance, the distance measurement position changes to the section of the plane identified by numeral 9-1 if, for example, the object to be photographed is at a very short distance away, and which distance is shorter than a distance corresponding to the section 8-1.
This relation, of course, varies with positional arrangement of the light projecting and receiving (sensitive) elements. Further, the image size of the projected light spot within the image plane changes with the focal length of the photo-taking lens system when the lens is a zoom lens system. In such a case, therefore, the distance measurement position changes to a greater extent as indicated by numerals 8-2, 9-2 and 10-2 in FIG. 2. Under these conditions, when the photographer peers into the view finder, he is unable to see the part to which distance measurement is being applied and thus tends to take a photograph with the lens focused on the wrong object. Therefore, there have been cases where poor photographs have resulted from the use of conventional distance measuring devices. Particularly in the case of a camera having a lens of high magnification such as an 8 mm camera or a video camera, the distance measurement position falls outside of the view finder because of the high magnification of the lens and also the base line length which is long due to the use of a high magnification lens. In such a case, therefore, the object to be photographed moves completely out of focus even if an image of the object is caught within the view finder. This has presented a serious problem.